Two sentence summary:
People were presented with the opportunity to live forever after writing a note that recounted their recent successes and various pleasures. After finishing the note, each participant was given a pill that afforded them immortality (and minty-fresh breath that is seemingly inherent to eternal life).

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How It Sorta Happens:
[I waddle up to a stranger. I’m carrying a clipboard and dressed like an adult.]

Me: Are you interested in living forever?

[The stranger pauses, acknowledges I exist.]

Me: Today and today only, I have a unique opportunity for you. If you write down everything good that is happening in your life, right here on this form. [Turns clipboard to them.] I can help you live forever. All you have to do is fill out this form—writing why you’ve decided on immortality… and then we give you a small white pill. Then, [I snap my fingers] you’re swimming in the fountain of youth.

From the event page:
"Are you happier than ever and ready to live forever? Do you want to spit in Death's face? Do you want to meet your great-great-great-great-great-great-great grandkids?

It's time for you to meet Father Time's sexy cousin, Xibulba®. This life changing drug will give you the chance to live forever.

This [Day], [Date], GreyRainbow™, a "global research-based biopharmaceutical company" will be hosting a day-long writing workshop on how to write a note to friends and family about why you've chosen to live forever. During the writing workshop, you will recount your recent victories over your lost keys, tight jar lids, and the one really stupid person you hate. It will be a love song to life. It is a ContraDeath™ note.

After completing the note, a Xibulba® (jä-b?l-bah) pill will be administered to each participant – which means that immortality literally will be in their hands."